1. Field of the Invention
The present invention relates to a nickel hydroxide positive active material for an alkaline battery and a nickel hydroxide electrode using the nickel hydroxide active material.
2. Description of the Related Art
A nickel hydroxide electrode comprising an active material as a compound mainly composed of nickel hydroxide has been used as a positive electrode for an alkaline battery such as a nickel-cadmium storage battery and a nickel-metal hydride storage battery. Such an alkaline storage battery is used as a power supply for small-sized portable electronic devices. In order to improve these electronic devices, alkaline storage batteries having an enhanced capacity have been desired. To this end, it is necessary that the nickel hydroxide electrode, which governs the discharge capacity of alkaline storage battery, have an enhanced high energy density.
As a nickel hydroxide electrode, there has heretofore been often used a sintered electrode. The sintered electrode is prepared by sintering a nickel powder on a porous punching metal to obtain a substrate, and then impregnating the substrate with an nickel hydroxide active material. However, such a sintered nickel hydroxide electrode is disadvantageous in that the substrate exhibits porosity as small as about 80%, and thus can be hardly impregnated with a large amount of an active material. Therefore, such a substrate is unfavorable for the enhancement of the energy density of nickel hydroxide electrode.
On the other hand, a non-sintered nickel hydroxide electrode obtained by retaining a nickel hydroxide active material in a three-dimensional porous metal material such as foamed nickel or fibrous nickel as substrate exhibits a substrate porosity as high as not less than 95%. Thus, it is favorable for the enhancement of the energy density of nickel hydroxide electrode as compared with the foregoing sintered nickel hydroxide electrode. Under these circumstances, studies of enhancement of capacity of alkaline storage batteries have been made mainly using non-sintered nickel hydroxide electrode.
The non-sintered nickel hydroxide electrode exhibits a low electrical conductivity of nickel hydroxide. Accordingly, it is necessary to incorporate a cobalt compound such as cobalt hydroxide, cobalt monoxide and cobalt suboxide or metallic cobalt as an electrically-conducting agent in the nickel hydroxide electrode or coat the surface of particulate nickel hydroxide with such a cobalt compound or metallic compound as disclosed in JP-A-62-117267 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d).
It is thought that the cobalt compound thus added is converted to cobalt oxyhydroxide having a high electrical conductivity by electrochemical oxidation during the first charging for formation. Then, it forms an electrically conductive network in the electrode to function as an effective electrically conducting agent in the nickel hydroxide electrode. The coating of the surface of particulate nickel hydroxide makes it possible to raise the area of contact of the cobalt compound with nickel hydroxide and hence add to the effect of enhancing the percent utilization of active material as compared with the single addition of the cobalt compound.
Nickel hydroxide, which acts as an active material for nickel hydroxide electrode in alkaline storage batteries, is oxidized during charging to form nickel oxyhydroxide. Further, it is reduced to nickel hydroxide during discharging. Nickel hydroxide is a material having a lower electrical conductivity than nickel oxyhydroxide. Thus, nickel hydroxide is disadvantageous in that it causes a drop of charge efficiency at the initial stage of charging, particularly during the first charging for formation.
In order to solve these problems, an approach has been proposed involving the uniform addition of nickel oxyhydroxide, which is a material having a higher electrical conductivity than nickel hydroxide, to a nickel hydroxide electrode material before the preparation of a nickel hydroxide electrode as described in JP-A-2-262245 and JP-A-2-234356.
However, it was found that the addition of nickel oxyhydroxide to a nickel hydroxide electrode having the conventional cobalt compound having a cobalt valence of 2 or less such as cobalt hydroxide and cobalt monoxide or metallic cobalt incorporated therein gives a smaller discharge capacity than calculated.
It is an object of the present invention to provide an improved nickel hydroxide active material for an alkaline battery and a nickel hydroxide electrode using the improved nickel hydroxide active material.
The inventors made studies of the reason for the above phenomenon. As a result, it was concluded that the cobalt compound having a cobalt valence of not more than 2, even if it partly has a cobalt valence of greater than 2 but lower than the valence of nickel hydroxide having a valence of greater than 2, partly undergoes oxidation by nickel oxyhydroxide, which has a high oxidizing powder, to form a high order cobalt compound that causes the maldistribution of cobalt compound in the nickel hydroxide electrode, making it impossible to sufficiently form a network of electrical conductivity of cobalt oxyhydroxide, which is considered required to maintain the electrical conductivity of the nickel hydroxide electrode. It was found on the basis of this conclusion that the previous formation of cobalt oxyhydroxide on the surface of particulate nickel hydroxide gives solution to the foregoing problems. The inventors further found a method for enhancing the discharge capacity of alkaline battery in the course of their studies.
According to the present invention, a nickel hydroxide positive active material for an alkaline battery comprises: nickel hydroxide powder having a nickel valence of greater than 2; and a cobalt compound having a cobalt valence of greater than 2, which is formed on the surface of said nickel hydroxide powder.
According to the present invention, an electrode is composed of a three-dimensional porous material retaining the above nickel hydroxide positive active material therein.
In accordance with the present invention, the percent utilization of active material in the nickel hydroxide electrode can be enhanced, making it possible to provide the alkaline battery with an enhanced capacity.